A lot of it is down to the confidence of the fish unless it is a canal just full of small 'uns.
On canals the larger fish tend to be shy and will often be the last to feed, often waiting until the small ones feed first.
Once their confidence is boosted they will then start.
How do you build their confidence? firstly use light tackle but make sure that your rod or elastic is balanced. There is no point in using a heavy test curve rod or heavy elastic with light lines as you will just snap off the minute you hook into something bigger.
In most cases 1.5lb hooklengths are more than adequate, try to use the 'double-strength' ones that are supposedly half the diameter compared to other lines.
A rod of 3/4lb test curve or no. 8 elastic with a pole is ample.
Then you need to feed the smaller fish little and often, maintaining a steady feed rate but not too much as you want to maintain interest not feed the fish up.
You will more than likely find that the larger fish are on the sidelines or underneath the little ones, so good idea is to toss in a small amount of loose feed and then follow it with your hookbait towards the outside of your feeding area.
Method in the madness is that the small ones will rush to the middle of the feeding area whilst the larger ones remain on the sidelines which is where your bait will be falling. The larger ones will see the samller ones feeding and be more tempted to join in.
If that doesn't work then get your hokbait on the bottom and feed over it. This will have the effect of feeding the little 'uns over your bait and the larger ones may well scour the bottom for food that has got past them.
there are several options to fish canals in winter
my favourite way if im going for 4hrs + is to fish 2 swims
1 on inside at 3to4 mts start on breadpunch feeding small walnut sized balls of liquidised bread every 5-10mins also every 3 to4 mins introduce 6/8 grains of hemp. start on a 22 hook & try different sizes of punch if you start to get better fish try hemp on hook.
2 on the far shelf/bank feed chopped worm/caster every 20mins after first hr try over this with a section of worm on hook this should bring you bonus fish.
you did not mention wether you use pole, whip or running line if you let me know ill give you some idea on rigs/setup
best of luck & welcome to the mad MDs